1658
HERBERT e. BROWNA K D
of several aromatics. These observations are summarized in Table IV. The logarithms of the partial rate factors are plotted against the standard electrophilic substituent constant^,'^ u+, in Fig. 1. With but one exception, p-fluoro, the data are correlated with excellent precision. Omission of the p-fluoro point yields the reaction constant for the ethylation reaction as -2.44.
Experimental Part Materials.-Gallium bromide,5 ethyl bromide5 and ethylene dichloride14 were prepared and purified following procedures previously described. The halobenzenes were samples purified by G01dman.l~ Authentic o-chloroethylbenzene was prepared by the Sandmeyer reaction from o-ethylaniline. m-Bromoethylbenzene was available from a n earlier study.16 The chlorination and bromination of ethylbenzene providing the ortho and para isomers were carried out by procedures previously utilized t o establish the isomer distribution in toluene.7J7 Relative Rates.-A solution of gallium tribromide in ethyl bromide was prepared with careful exclusion of moisture. (13) H. C. Brown and Y. Okamoto, J . A m . Chem. SOC.,80, 4929 (1958). (14) F. R. Jensen, G. hlarino and H. C. Brown, i b i d , 81, 3303 (1959). (15) H. C. Brown and G. Goldman, i b i d . , 84, 1650 (1962). (16) H. C. Brown, 1. D. Brady, M. Grayson and W. H. Bonner, ibid., 79, 1897 (1957). (17) H.C. Brown and L. M. Stock, ibid., 79, 5175 (1957).
[CONTRIBUTION FROM THE
RICHARD B.
WETHERILL
GIANLORENZO L I A R I N 0
Yol. s4
An appropriate small quantity of this standard solution, usually about 1 ml., was taken up in a hypodermic syringe and introduced into 50 ml. of a solution of the halobenzene and benzene in ethylene dichloride a t 25". After a n appropriate reaction period, 2 to 10 hours in different experiments, the reaction mixture was queuched in ice-water. The ethylene dichloride layer was separated, washed, and dried. The solution was analyzed on a Perkin-Elmer Fractometer (model 154C) equipped with an integrator utilizing R 2-m. column with tricresyl phosphate as the liquid phase on Celite. The concentration of each aromatic was evaluated; z.e., benzene in the ethylated mixture was determined relative to benzene in the original solution and each halobenzene similarly determined. I n view of excellent reproducibility in sampling techniques, lyi, a n internal standard was n o t employed, The temperatures utilized for the analyses were: benzene, 58"; fluorobenzene, 55 ; chlorobenzene, 104'; bromobenzene, 130". The results are summarized in Table I. Isomer Distribution.-In a typical experiment a n ethylene dichloride solution of bromobenzene, 0.7 ilf, was treated with gallium tribromide, 0.04 M , and ethyl bromide, 0.5 M . After 5 hours, the reaction mixture was quenched and analyzed on a 4-m. column of tricresyl phosphate on Celite. For fluorobenzene and chlorobenzene, ethylene dichloride was eliminated and excess aromatic was employed as the solvent. Thus fluorobenzene was treated with ethyl bromide, 0.5 M , and gallium bromide, 0.04 M . After 2 hours, the reaction mixture was quenched, dried, and analyzed on a Perkin-Elmer Fractometer (model No. 154D) equipped with a 50-m. polypropylene glycol column and a flame ionization detector. T h e isomer distributions obtained are summarized in Table 11.
LABORATORY O F PURDUE
USIVERSITY,
LAFAYATTE, ISD.]
Relative Rates and Isomer Distributions in the Aluminum Chloride-Catalyzed Acetylation of the Halobenzenes in Ethylene Dichloride. Partial Rate Factors for the Acetylation Reaction1 BY HERBERT C. BROWNAND GIANLORENZO MARINO' RECEIVED NOVEMBER 10, 1961 The rate of aluminum chloride-catalyzed acetylation of chlorobenzene relative to benzene in ethylene dichloride a t 25' is 0.0209. Under these conditions the reaction yields 0.5y0m- and 99.570 p-chloroacetophenone. These results lead t o the partial rate factors mfClO.0003 and p f c l 0.125. Comparison of the rate of acetylation of fluorobenzene and bromobenzene ~ C ~ and H ~ kBrC&/k~H6 0.0140. In both cases essentially relative t o chlorobenzene leads to the relative rates, ~ F C ~ I I ~ / 0.252 lOOy0substitution in the para position is observed. These results provide the partial rate factors, piF 1.51 and piBr 0.0%. T h e negligible substitution in the ortho position is in accord with the very large steric requirements of the acetylation reaction. noted previously in the acetylation of toluene The order pfF > > piBragrees with the relative conjugative abilities of these halogens and with their relative effectiveness in facilitating reaction in mercuration, ethylation and t-cumyl solvolysis. T h e observation t h a t pfF is greater than unity is in agreement with the results for mercuration and t-cumyl chloride solvolgsis, but not with the behavior of p-fluoro in the ethylation reaction. The availahle data on the acetylation of monoiubstituted benzenes provide an excellent linear free energy correlation with the electrophilic substituent constants, u+, derived from the solvolysis of the t-cumyl chlorides.
Introduction reactions have been reported. These reactions esThe acetylation reaction has previously proved hibit a relatively low selectivity, with Sf 1.014 and to be a convenient tool for the investigation of direc- 0.587, respectively. In the present paper, data for tive effects in t ~ l u e n et-b~tylbenzene,~ ,~ anisole,j bi- the acetylation reaction, one of higher selectivity, phenyl6 and fluorene.6 In previous papers of this Sf 2.192, are presented. I n the following paper the group, electrophilic substitution data for the halo- more selective halogenation reactions are examined7. benzenes in the r n e r c ~ r a t i o nand ~ ~ e t h y l a t i ~ n ~and ~ in the final paper7d the applicability of the Selectivity RelationshipsV9 for the correlation of the (1) Diiective Effects in Aromatic Substitution. LII. available data on the electrophilic substitution of (2) Post-doctorate research associate, 1957-1959, on project no. the halobenzenes is considered. A T ( l l - 1 ) - 1 7 0 supported by t h e Atomic Energy Commission. (3) H. C. Brown, G. Marino and L. hl. Stock, J . A m . Chem. Soc., Several studies of the acetylation of the haloben81, 3310 (1959). zenes have been reported. Thus, the acetylation of (4) H. C. Brown a n d G. Marino, i b i d . , 81, 5611 (1959). chlorobenzene, either in carbon disulfide1°-12 or ( 5 ) H. C. Brown and G. Marino. unpublished research. (6) H. C. Brown and G. Marino, J . A m . Chem. Soc., 84, 1236 (1962). (7) (a) E. C. Brown and G. Goldman, ibid., 84, 1650 (1962); (b) H. C. Brown and A. H. Neyens, ibid., 84, 1655 (1962); (c) L. M. Stock
a n d F. W. Baker, ibid., 84, 1661 (1962); (d) L. M. Stock and H. C. Brown, i b i d . , 84, 1668 (1962). ( 8 ) H. C. Brown and K. L. Kelson, i b i d . , 75, 6292 (1953). (9) H. C. Brown and C. W. McGary, Jr., i b i d . , 77, 2300 (1955).
CATALYZED ACETYIATION OF HALOBEKZENES
May 5, 1962
TABLE I HALOBENZENE TO BENZENE REACTIVITY RATIOS FOR THE ALUhfIxUM CHLORIDE-CATALYZED DICHLORIDE AT 25.0' ICHCOC1,AlClsl
ACETYLATION I N
Concentrations, M Starting materials
7
[ClHsl
[FC&s]
[CIC~HS]
[BrCsHa]
1659
-Products[XCsHCOCHa] [ C I C ~ H ~ C O C H J ]
Exptl. relative ratesa
ETHYLENE Reactivity ratio.
kcansx/kcs~a
0.0191 0.0191 ,0198 .0198 ,0238 .0238 ,150 Mean value ,0209~ 0,0600 0.0524 0.223 0.0239 0.0116 1l.j 0.239 0.0600 0.0800 0.324 0.0135 0.00461 12.7 0.266 Mean value 0.252' 0,0800 0.133 0.134 0.0162 0.0247 0.628 0.0131 0.0600 0.136 0.117 0.00132 0.00696 0.710 0.0118 htean value 0 .o l i o d Comparisons of chlorobenzene t o tlenzene. fluorobenzene t o chlorobenzene and hromohenzene to chlorobenzene, respectively. X = C1. X = F. d X = Br. 0,200 ,150 ,100
0.300 ,225
0.900 ,675 .900
0.1810 ,1375 ,063
0.0159 .0125 .0121
chl~robenzenel~ as solvent, has been reported to relationship I give p-chloroacetophenone exclusively. Similarly, p-bromoacetophenone is reported to be the sole product from the acetylation of b r o r n ~ b e n z e n e . ~ ~ ~ l ~ - ~ ~ McDuffie and Dougherty determined the rates In this expression, .A0 and Bo are the initial conof acetylation of chlorobenzene and bromobenzene ceiitration of benzene and the halobenzene, respecrelative to benzene by a competitive procedure.17 tively. The symbols XAand Xg refer to the final The values obtained are chlorobenzene, 0.031, and concentration of product acetophenone and the isobromobenzene, 0.024, relative to benzene, 1.OO. meric haloacetophenone. The first experiments Unfortunately, the conditions employed by Mc- established the relative rate of acetylation of chloroDuffie and Dougherty could result in a heterogene- benzene compared to benzene. In subsequent work, ous reaction. Moreover, the observed toluene to i t proved more convenient to compare fluorobenbenzene rate ratio," 13.3, was in serious error with zene and bromobenzene with chlorobenzene rather the value, 128, obtained for the relative rate under than in direct competition with benzene. homogeneous conditions in ethylene di~hloride.~ The relative rate data are assembled in Table I. Analysis of reaction mixtures which were mainMore recently Smeets and Verhulst examined the tained for periods of time suficient to complete the kinetics of the acylation of a number of aromatic compounds including chlorobenzene and bromo- reaction revealed the yields of acetophenones were benzene.18 They followed the reaction by titra- not quantitative. Yields were only S0-6Sy0 of the tion of the hydrogen chloride liberated in the course theoretical. This is in contrast with the results for of the reaction. Cnfortunately, the reaction with more active aromatics which provided essentially benzene proved to be too fast for this technique. quantitative conversions to the ace top hen one^.^-^ I t is known that the 1 : 1 complex of aluminum A careful review of these studies suggested the possibility of rather large errors or uncertainties. chloride and acetyl chloride, CH3COC1.A1C13, is Accordingly, i t appeared desirable to re-examine somewhat unstable. Solutions of aluminum chlothe acetylation of the halobenzenes under standard ride in acetyl chloride are reported to undergo conditions employing gas chromatography and in- secondary reactions. l g P z 1 I t was observed that frared spectroscopy for the analysis of rates and freshly prepared solutions of the complex in ethylene dichloride evolved hydrogen chloride slowly a t isomer distributions. 2.5'. The decomposition is sufficiently slow as to be unimportant in the case of the more active aroResults m a t i c ~ . ~ -The ~ side-reaction, however, becomes Relative Rate Determinations.-Mixtures of the significant with the less reactive halobenzenes. halobenzenes and benzene in ethylene dichloride Fortupately, this side-reaction does not influence were treated with a freshly prepared solutio11 of the relative rates. The latter results are based on a aluminuni chloride-acetyl chloride in ethylene di- coniparison of the two acetophenones produced in chloride. After an appropriate time, the reaction the competitive reactions. The data reveal reasonmixture was quenched, washed, dried, and analyzed by gas chromatography for the products. The relaTABLE I1 tive reactivities were calculated by means of the PARTIAL ]
